Method to operate an incoherently emitting radiation source having at least one dielectrically impeded electrode

ABSTRACT

A method to operate an incoherently emitting radiation source, in particular a discharge lamp, which transmits UV, IR or VIS radiation. The discharge is generated by means of a train of voltage pulses, interrupted by idle times, inside a discharge vessel; electrodes dielectrically impaired on one or both ends can be used. By a suitable choice of the filling, the electrode configuration, the sparking distance, the type and thickness of the dielectrics, the time-dependent voltage amplitudes, and the pulse and idle times, efficiencies in UV generation of 65% and more are attained.

FIELD OF THE INVENTION

The invention relates to a method for operating an incoherently emittingradiation source, and more particularly to ultraviolet, infrared andvisible light radiators, such as discharge lamps.

BACKGROUND

A discharge is generated within a discharge vessel. The discharge vesselhas a dielectric layer disposed between at least one electrode and thedischarge, and for this reason this type of discharge is also known as asilent, or quiet or dielectrically impaired discharge, or barrierdischarge. Incoherently emitting radiation sources include ultraviolet(UV) and infrared (IR) radiators, as well as discharge lamps that inparticular radiate visible light. The invention is suitable for bothlow-pressure and high-pressure gas fillings and for all gas pressuresthat are in the range between low and high pressure.

The excitation of such discharges is typically done with the aid of analternating current, as disclosed for instance in the German PatentDisclosures DE 40 22 279 , Eliasson et al., and DE 42 03 594, Kim and inU.S. Pat. No. 5,117,160, Konda et al. The excitation frequency there isselected within the range between the frequency of thecommercial-technical alternating current and several megahertz (DE 40 22279), or between 20 and 100 kHz (U.S. Pat. No. 5,117,160).

The disadvantage of this mode of operation is that the desired radiationyields are relatively low, at technologically relevant power densities.Typical UV efficiencies are between 10%, for surface power densities of1 kW/m² and 15% at 10 W/m² ; see "3. Tagung des Arbeitskreises UV und IRam Lichttechnischen Institut der Universitat Karlsruhe" ["3rd Conferenceof the UV and IR Study Group at the Light Technology Institute,University of Karlsruhe"] on Oct. 7, 1992, and "Dielectric BarrierDischarges: An Unusual Light Source", M. Neiger, LTI, University ofKarlsruhe, Sixth International Symposium on the Science and Technologyof Light Sources, Budapest, 1992.

The Invention

The object of the invention is to improve the efficiency of the desiredgeneration of radiation substantially.

Briefly, the fundamental concept of the invention is based on the factthat a dielectrically impeded, or impaired discharge, also referred toas a barrier discharge, is operated by being repetitively pulsed, sothat the successive electrical energy introductions are interrupted bytime periods T_(On) --hereinafter called "idle times"--even at highpower density in the individual discharge. The lengths of the individualperiods of time are a function of the requirement that the introductionof energy, or more precisely the introduction of the effective power, beessentially ended as soon as the introduction of further electricalenergy has resulted in a less-efficient conversion into the desiredradiation the idle time is ended as soon as the gas filling has relaxedonce again enough, respectively, to enable renewed exciting forefficient emission for the desired radiation. Thus, on a temporalaverage the radiation efficiency is optimized. In this way, efficienciesof 65% and more, for example, in the conversion of electrical energyinto UV radiation can be attained, which is an increase multiple timesover the conventionally operated, dielectrically impaired discharge.

In the normal situation, this involves a train of identical voltagepulses, or voltage pulses that merely change their polarity; the totalnumber n of voltage pulses is in principle unlimited. For special cases,however, a train of voltage pulses that regularly vary can be used aswell. Finally, the pulse train can also be entirely irregular (forexample, in special effect lighting, where a plurality of pulses arecombined into a cluster such that a particular light effect that isapparent to the human eye is created).

During the pulse periods T_(Pn), a voltage pulse U_(Pn) (t) is appliedbetween the electrodes, and effective power is introduced. Its courseover time is not in principle fixed; it can be chosen from variousforms, such as:

a) unipolar forms; that is, the voltages do not change their sign duringthe pulse times T_(Pn) ; this includes, among others, trapezoidal,triangular, and arclike-curved voltage pulses, especially parabolicvoltage pulses and sinusoidal half-waves; both positive and negativevalues are suitable (see FIG. 6a, which by way of example shows onlynegative values);

b) bipolar forms; that is, the voltages do change their sign during thepulse times T_(Pn) ; the forms can begin with a positive or a negativesign. Examples of this are both half-waves of one sinusoidal wave; twoimmediately sequential triangles of opposite sign; two immediatelysuccessive "squares" or trapezoids of opposite sign; the edges may havedifferent rise or fall times (see FIG. 6b); and

c) the succession over time of a few (preferably 2 or 3) elements fromparagraphs a and b, in which the voltages U_(Pn) (t) can assume greatlyvarying values, and in particular briefly the value of 0, so thatindividual elements can also be separated even by time periods in whichthe voltage has the value zero (see FIG. 6c). In particular, theindividual elements can repeat.

FIGS. 6a-c by way of example show only one selection of possible voltageshapes. Beyond this, a great number of further shapes is possible. Inparticular, electrical signals in practice always have finite rise andfall times, and overswings and underswings, which is not shown in FIGS.6a-c.

The demand made in terms of of the voltage shape during the idle timesT_(0n) is that the voltage U_(0n) (t) be selected such that essentiallyno effective power introduction occurs. Correspondingly low voltagevalues, which are less than the reignition voltage, can last for longertimes, optionally the entire idle time T_(0n). It is not precluded thatvoltage peaks may also occur briefly, or in other words forsubstantially less than the pulse time T_(Pn), during the idle time.

Typical absolute values for U_(Pn) are a few kilovolts. U_(0n) ispreferably in the vicinity of 0 V. The values of T_(Pn) and T_(0n) aretypically in the microsecond range, and normally T_(Pn) is markedlyshorter than T_(0n).

The operating conditions according to the invention for the dischargeare attained essentially by a suitable choice of the excitationparameters T_(Pn), T_(0n) and voltage amplitude U_(Pn) ; these variablesare adapted suitable to one another for particularly efficientoperation. The pulse shape also plays a role.

In an individual case, the values to be selected for the threeexcitation parameters T_(Pn), T_(0n) and U_(Pn) (t) are dependent on thedischarge geometry, the type of gas filling, and the gas pressure, aswell as the electrode configuration and the type and thickness of thedielectric layer. If the discharge is taking place under the operatingconditions of the invention, then the yields in the desired radiationassume an optimum value.

The rates of the impulse processes that occur for given lamp fillings inthe discharge, and consequently the radiation generation as well, aredetermined essentially by the electron density n_(e) and the energydistribution of the electrons. The operating method according to theinvention makes it possible for these time-dependent variables to beadjusted optimally for radiation generation by means of a suitablechoice of T_(Pn), T_(0n) and the voltage amplitude U_(Pn) or the pulseshape.

In comparison with the alternating voltage mode of operation, theinvention intentionally utilizes one additional parameter, which is the"idle time" T₀, with which for the first time, even at high powerdensities, influence can be purposefully exerted upon the chronologicaland three-dimensional course of the charge carrier density and on theenergy distribution function. In the prior art, in which alternatingvoltage is used, a purposeful influence can be exerted on thesevariables only in a very limited sense by way of the frequency. Thepresent invention makes it possible for the first time for theefficiency of dielectrically impaired discharges to be purposefullyincreased with industrially valuable power densities such thatalternatives to conventional radiation sources are provided.

The operating conditions of the invention can be recognized from thefact that between the electrodes, instead of differently embodied,typically filamentlike or coillike discharge structures, a number ofdischarge structures occur which are identical in plan view or in otherwords at right angles to the discharge and are similar in shape to adelta; they each wider in the direction of the (instantaneous) anode.Since these discharge structures are preferably generated at repetitionfrequencies in the kHz range, the observer perceives only an "average"discharge structure that corresponds to the resolution over time of thehuman eye, in a manner similar to that shown by the photograph in FIG.9a. In the case of alternating polarity of the voltage pulses of adischarge that is dielectrically impaired on two ends, the visualappearance is a superposition of two delta-shaped structures. Forexample, if two elongated electrodes, which may be dielectricallyimpeded, or impaired on one or both ends, are parallel facing oneanother, then the various discharge structures appear to be orientedtransversely to the elongated electrodes, in rows next to one another(see FIGS. 9a, b). With a suitable choice of parameters, for example ata suitably low pressure, the arrangement of individual structures inrows leads to a single, diffuse-looking discharge which may be termed acurtain-like discharge; The discharge structures may for example beobserved directly in transparent lamp bulbs.

One significant advantage of the invention resides in the particularstability of the individual discharge patterns, or structure, comparedwith a variation of the electrical power density introduced into thedischarge vessel. If the amplitude U_(Pn) of the voltage pulseincreases, then the various discharge patterns, or structures do notchange their basic shape. Once a threshold value has been exceeded,other similar patterns, or structures are created from one of thedischarge patterns. An increase in the introduced electrical power byincreasing the amplitude of the voltage pulses, accordingly leadsessentially to an increase in the number of individual dischargestructures described, while the quality of these structures, and inparticular their external appearance and their efficient radiationproperties, remains unchanged.

This behavior makes it possible for the first time for the electricalpower that can be introduced into a given discharge volume to besuitably increased still further by using more than two electrodes,which optimally utilize the discharge volume. For example, a pluralityof external electrodes can be located symmetrically on the outer wall ofthe discharge vessel, opposite one central internal electrode locatedinside the discharge vessel. Thus with the number of externalelectrodes, the maximum radiation power that can be extracted from thevolume of the discharge vessel can be increased, since the dischargestructures, beginning at the central inner electrode, burn in thedirections of the respective external electrodes and thus increasinglyfill the volume of the discharge vessel, given correspondingintroduction of power.

In addition to this option, if the electrodes are arranged axiallyparallel a further advantage is attained in that the electrical powerand the light current vary in proportion to the length of the dischargevessel.

Since in this case the electrical field is essentially at right anglesto the longitudinal axis of the discharge vessel, the length of thedischarge vessel can be increased virtually arbitrarily, without acorresponding rise in the requisite ignition voltage of the kind that isusual in a conventional tubular discharge lamp.

For a rated power, with this type of discharge, both the volume of thedischarge vessel and the number of electrodes, or planes in which thedischarge structures burn, must therefore be taken into account. For atubular lamp 50 cm in length, 24 mm in diameter, and with xenon as thefilling gas, typically 15 W of electrical effective power can beintroduced per "discharge plane".

If T_(Pn) and/or T_(0n) and/or U_(Pn) (t) are not chosen suitably, thenthin, brightly lighted "discharge filaments" occur stochastically, whichare more or less sharply demarcated from the gas chamber. At the expenseof the discharge structures according to the invention, they can extendover wide regions within the discharge vessel, as can be seen from thephotograph in FIG. 10b. These "discharge filaments" are thus markedlydifferent visually both in shape and in their spectral radiationdistribution from the discharge shape of the operating conditions of theinvention and are undesirable, since they concentrate the currenttransport within small cross-sectional areas, resulting in increasedcharge carrier densities, associated with increased quench rates amongother results, and consequently the efficiency in generating the desiredradiation decreases.

From this phenomenology, a general prescription for attaining thesuitable values for U_(Pn) (t), T_(Pn) and T_(0n) for the mode ofoperation according to the invention can be derived. After the ignitionof the discharge, U_(Pn) (t), T_(Pn) and T_(0n) should be selected suchthat the desired electrical discharge is introduced under the operatingconditions of the invention; that is, the above-described dischargestructures are visible. Surprisingly, it has in fact been found thatprecisely in the presence of these discharge structures, the electrondensity and the energy distribution function of the electrons bothassume values that minimize the loss processes.

Each of the aforementioned three operating parameters influences notonly the chronological and three-dimensional structure of the chargecarrier densities but also the energy distribution function of theelectrons. Since their various influences on the aforementionedvariables are variously pronounced, the choice of one parameter fixes arough range of values for the remaining parameters for attaining theefficient discharge mode.

Typical values for the amplitude U_(Pn) of the voltage pulses are in therange between approximately 0.01 and 2 V per centimeter of sparkingdistance and per pascal of fill pressure; the pulse times T_(Pn) and theidle times T_(0n) are on the order of magnitude of approximately 1 ns to50 μs, and approximately 500 ns to 1 ms, respectively. For the mode ofoperation of the invention, the operating pressure is advantageouslybetween 100 Pa and 3 MPa. In the medium-pressure range (for instance, 10kPa), this preferably means an amplitude U_(Pn) of the voltage pulses inthe range between 100 V and 20 kV per centimeter of sparking distance.In the high-pressure range (for instance, 1 MPa), this preferably meansan amplitude U_(Pn) of the voltage pulses in the range between 10 kV and200 kV per centimeter of sparking distance.

For the sake of electrical safety, the external electrodes arepreferably connected to ground potential and the inner electrode ispreferably connected to the high voltage. Extensive protection againsttouching parts that carry voltage is thereby possible. The dischargevessel, including the electrodes, can also be disposed inside anenveloping bulb. Touch protection is thereby attained even if theexternal electrode or electrodes are not connected to ground potential.Any materials capable of carrying current, including electrolytes, canbe used as the conductive electrode material.

For the discharge dielectrically impeded on one end--that is, at leastone dielectrically unimpeded electrode is located inside the dischargevessel, in the gas chamber--it is moreover compulsory for this internalelectrode, at the beginning of the pulse period, to be given a negativepolarity (except for possible needle-shaped preliminary pulses that areinsignificant in terms of the power introduction), compared with thedielectrically impeded, or electrode (inside or outside the dischargevessel). After that, the polarity can alternate during the pulse period.

The mode of operation according to the invention is also suitable fordischarges dielectrically impeded on both ends (all the electrodes areseparated from the discharge by a dielectric, which may also be thedischarge vessel itself), without in principle having to be changed orhaving to lose its advantageous effect. In the event that all theelectrodes are dielectrically impeded, or the chronological successionof the polarity and the polarity itself play no role at all.

In principle, the electrodes can be located either all outside the gaschamber, for instance on the outer surface of the discharge vessel, orelse a certain number of them may be located outside and a certainnumber inside, and they can all also be located inside the dischargevessel, in the gas chamber. In this last case it is necessary for atleast one of them to be coated with a dielectric and in process to begiven a polarity opposite that of the other electrodes.

Particular, in the event that aggressive media are located inside thedischarge vessel, it is advantageous if none of the electrodes havedirect contact with the medium, because then corrosion of the internalelectrode or electrodes can be effectively prevented. This can beaccomplished either by locating all the electrodes outside the dischargevessel, or by surrounding those electrodes located within the dischargevessel with a dielectric layer.

In the invention, large-area electrodes are dispensed with.

The shading of the radiation by the electrodes is very slight. For thedielectrically impaired electrodes, the ratio of the electrode area incontact with the dielectric to the circumference of this electrode areais advantageously as low as possible. In an especially preferredembodiment, the dielectrically impaired electrodes are formed as narrowstrips applied to the outer wall of the discharge vessel. Gridlikeexternal electrodes are also suitable, such as grid networks, perforatedplates, or the like. To enable optimal utilization of the volume of thedischarge vessel, the internal electrode preferably has the smallestpossible length in the direction of the discharge. In an especiallypreferred embodiment, the internal electrode is embodied as a rod.

The discharge impaired on one or both ends makes it possible to achievea great number of possible discharge vessel geometries, in particularincluding all those disclosed for conventionally operated dielectricallyimpaired discharges in the following patent disclosures, by way ofexample: EP-A 0 385 205, European Patent 0 312 732, EP-A 0 482 230, EP-A0 363 832, EP-A 0 458 140, EP-A 0 449 018 and EP-A 0 489 184.

In discharge vessels with small cross sections, the electrodes shouldpreferably be located such that the distance between the correspondinganodes and cathodes is as great as possible. For example, forcylindrical discharge vessels of small cross section, the internalelectrode is preferably located eccentrically inside the dischargevessel, and the external electrode is fixed diametrically opposite it onthe outer wall. The lengthening of the discharge paths can beadditionally reinforced by subdividing the electrodes. To that end, theinternal and external electrodes have two different regions inalternation, within which the discharge begins and is suppressed,respectively. The electrodes are then located such that two differentregions each face one another. This suppresses radial dischargestructures. The discharge inside burns obliquely to the next adjacentregion of the opposite electrode. This can be accomplished for instanceby having the electrodes have alternating regions with an additionaldielectric layer.

In the case of larger cross sections, the internal electrode ispreferably located centrally inside the discharge vessel, andadvantageously a plurality of external electrodes are fixed on the outerwall, distributed symmetrically over its circumference.

In principle, shape of the discharge vessel need not be specified in anycompulsory way. Depending on the intended application, the vessel wallsmust be of materials that have the requisite transparency--at leastwithin one aperture--for the desired radiation. Suitable dielectricbarriers for the high voltage employed are puncture-proof, electricallyinsulating materials (dielectrics) such as borosilicate glasses--forexample, DURAN® (made by Schott), quartz glass, Al₂ O₃, MgF₂, LiF,BaTiO₃, and so forth. The discharge structure can be varied by means ofthe type and thickness of the dielectric. Particularly, sufficientlythick dielectrics with sufficiently low relative dielectric constantsare suitable for reinforcing the development of the discharge structuresaccording to the invention with comparatively low electron densities, orin other words to avoid the development of undesired dischargestructures with high electron densities and current densities. Insimpler terms, this is the result on the one hand of the fact that thelocal voltage drop, caused by a displacement current density, across thedielectric is proportional to the thickness of the dielectric and isinversely proportional to the dielectric constant thereof. On the otherhand, the voltage drop at the dielectric counteracts an increase in thecurrent density.

The spectral composition of the radiation depends substantially on thegas filling and may for instance be in the visible, IR or UV range. Asthe gas filling, suitable examples in principle are all fillings thatcan be used for conventionally operated dielectrically impaireddischarges as disclosed for instance in German Patent Disclosure DE-OS40 22 279 or European Patent Disclosures EP-A 0 449 018, EP-A 0 254 111,EP-A 0 324 953, and EP-A 0 312 732, as well as fillings that are alreadyused in excimer or exciplex lasers (for instance: I. S. Lakoba and S. I.Yakovlenko, "Active media of exciplex lasers (review)", SOV, J. QuantumElectron. 10 (4), April 1980, pp. 389, and C. K. Rhodes, Editor,"Excimer Lasers" Springer, 1984). These include among others, noblegases and mixtures thereof, mixtures of noble gases with halogens orhalogen compounds, metal vapors and mixtures thereof, mixtures of noblegases with metal vapors, mixtures of noble gases with metal vapors andhalogens or halogen compounds, and also individual ones or combinationsof the following elements, which can also be added to the aforementionedfillings: hydrogen, deuterium, oxygen, nitrogen, nitrogen oxides, carbonmonoxide, carbon dioxide, sulfur, arsenic, selenium and phosphorus. UVgeneration in excimer discharges, which is highly efficient because ofthe mode of operation of the invention, in particular opens up thefurther field of application of UV high-power radiators, which ismentioned for example in EP-A 0 482 230. Among other things, this fieldincludes such photochemical processes as hardening photoresists,altering surfaces, disinfecting drinking water, or the like, andbreaking down pollutants by UV radiation in environmental technology.Particularly for these last-named fields of use, it can be advantageousfor the discharge to take place in the immediate vicinity of the mediumto be irradiated, or in other words for a hermetically sealed dischargevessel not to be used, in order to avoid an attenuation of theshort-wave portion of the radiation by the vessel walls. Particularly inthe generation of UV or VUV radiation, a further decisive advantagearises, which is the high UV yields attainable with the mode ofoperation of the invention: in contrast to UV or VUV radiators ofcomparable radiation densities found in the prior art, it is possible todispense with cooling by water. Another preferred application islighting, in which the UV radiation by means of suitable luminoussubstances is converted into the visible range of the electromagneticspectrum.

Further advantages of the invention are as follows: No external powerlimitation is necessary; the lamp is dimmable; it is possible to operatea plurality of lamps in parallel with only a single voltage supply; andhigh efficiency of the generation of radiation is attained, at the sametime at the power densities required in lighting technology areobtained.

In a preferred embodiment of the invention, the discharge vessel isprovided with a layer of luminous substance, in order to transfer thelight generated upon the discharge to particularly suitable spectralranges. A luminous substance coating can be used both with low-pressureand with high-pressure lamps. Luminous substances or mixtures known perse can be used for this. For fluorescent lamps, a combination of blue,green-and red-emitting luminous substances has proven especiallysuitable. One suitable blue luminous substance is in particular thebarium magnesium aluminate activated with divalent europium (BaMgAl₁₀O₁₇ :Eu²⁺). Terbium- or manganese-activated luminous substances areespecially suitable as the green component. Examples areterbium-activated yttrium oxide silicate (Y₂ SiO₅ :Tb) or lanthanumphosphate (LaPO₄ :Tb), or zinc silicate or magnesium aluminate activatedwith divalent manganese (Zn₂ SiO₄ :Mn or MgAl₂ O₄ :Mn, respectively).Advantageous red components are found among the luminous substancesactivated with trivalent europium, such as yttrium oxide (Y₂ O₃ :Eu³⁺)or borates of yttrium and/or gadolinium. Specifically, these are YBO₃:Eu³⁺, GdBO₃ :Eu³⁺, and the mixed borate (Gd,Y)BO₃ :Eu³⁺.

For lamps with a warm light color, the proportion of blue component canbe reduced or optionally left out entirely--in accordance with theprocedure already known for conventional fluorescent lamps.

For lamps with special color reproduction properties, components thatemit in the blue-green spectral range are suitable, examples beingluminous substances that are activated with divalent europium. For thisapplication, strontium borophosphate Sr₆ BP₅ O₂₀ :Eu²⁺ is preferred.

The invention in particular makes a breakthrough in the field offluorescent lamps. For the first time, it has been possible to dispensewith the mercury filling and nevertheless attain internal UVefficiencies that match those of conventional fluorescent lamps. Incomparison with conventional fluorescent lamps, the following additionaladvantages are also obtained. Problem-free cold starting is possiblewithout any influence of the ambient temperature on the light currentflux and without blackening of the bulb. Moreover, no electrodes (suchas glow cathodes with emitter paste) that limit the service life, noheavy metals, and no radioactive components (glow igniters) are needed.Unlike incandescent lamps and discharge lamps that have heated cathodes,the radiation is also emitted without notable delay, immediately afterthe application of the operating voltage to the electrodes. (The delayin illumination of the pure discharge is approximately 10 μs, includingluminous substance, approximately 6 ms. By comparison, the response timeof an incandescent bulb is in the range of approximately 200 ms. This isespecially advantageous for use in traffic light systems, traffic signsand signal lights.

DRAWINGS

The invention will be described in further detail below in terms ofseveral exemplary embodiments. The drawings, highly schematically, show:

FIG. 1, partly in section, a longitudinal view of an embodimentaccording to the invention of a discharge lamp in rod form, which can beoperated by the novel method;

FIG. 2a, the cross section along the line A--A of the discharge lampshown in FIG. 1;

FIG. 2b, the cross section through a further embodiment of a dischargelamp according to the invention;

FIG. 2c, the cross section through a further embodiment of a dischargelamp according to the invention;

FIG. 3a, a schematic view of the shape, preferred according to theinvention, of the voltage between the cathode and anode of the dischargelamp, dielectrically impaired on one end, shown in FIG. 1;

FIG. 3b, a schematic view of a shape of the voltage that can be usedonly for the operation according to one of the invention of dischargelamps dielectrically impaired on both ends;

FIG. 4a, partly in section, the plan view of a further embodimentaccording to the invention of a discharge lamp in the form of an arearadiator, which can be operated by the novel method;

FIG. 4b, the cross section through the discharge lamp shown in FIG. 4a;

FIG. 5a, the side view of a further embodiment according to theinvention of a discharge lamp, in the form of a conventional lamp withan Edison screw-type base, which can be operated by the novel method;

FIG. 5b, the cross section along the line A--A od the discharge lampshown in FIG. 5a;

FIG. 6a, a schematic view of several unipolar shapes of voltage pulsesUp(t) according to the invention having negative values;

FIG. 6b, a schematic view of several bipolar forms of voltage pulsesUp(t) according to the invention;

FIG. 6c, a schematic view of several shapes according to the inventionof voltage pulses Up(t), generated by combining individual elements ofFIG. 6a and FIG. 6b;

FIG. 7, measured shapes over time of the voltage Up(t), current I(t) andpower P(t)=U(t)·I(t) in the mode of operation according to the invention(173 hPa Xe, pulse frequency: 25 kHz);

FIG. 8, a view corresponding to FIG. 8, with a modified time axis;

FIGS. 9a, b, photographs of discharge pattern or structures according tothe invention;

FIGS. 10-d, photographs of the transition to undesired dischargepatterns or structures.

DETAILED DESCRIPTION

The invention can be described in an especially simple embodiment inconjunction with FIG. 1, which shows in a partly sectional longitudinalview, a medium-pressure discharge lamp 1, which is filled with xenon ata pressure of 200 hPa. Within the cylindrical glass discharge vessel 2,having a length of 590 mm, a diameter of 24 mm, and a wall thickness of0.8 mm, which defines a longitudinal axis, there is an axially parallelinternal electrode 3 in the form of a rod of special steel, 2.2 mm indiameter. Located outside the discharge vessel 2 is an externalelectrode, which comprises two two-millimeter-wide strips 4a, b ofconductive silver, which are axially parallel and are conductivelyconnected with the voltage supply. The individual conductive silverstrips 4a, 4b may, as shown in the present exemplary embodiment, bejoined together by a metal ring 4c and thus connected together and inturn in contact with the supply voltage. Care must be taken that themetal ring 4c be shaped sufficiently narrowly so as not to impede thedischarge. In a variant, the conductive silver strips 4a, b can also beconnected separately to the supply voltage. The internal electrode 3 iselectrically conductively contacted with a bail-shaped power lead 14.The power lead 14 is carried to the outside via a crimp 15, which isjoined in gas-tight fashion to the discharge vessel 2 by means of adished melt mount 16.

In a variant of this exemplary embodiment, the discharge vessel has anenlarged diameter, for instance in the form of a bead, in region of themetal ring. This prevents the occurrence of interfering parasiticdischarges in this region. In an especially preferred variant of theabove embodiment, the rodlike internal electrode is rigidly joined tothe first dished melt mount only on one end. Its other end is guidedloosely in a cylindrical tube secured centrally and axially to thesecond dished melt mount--in a manner similar to a fit with clearance.This has the advantage that the internal electrode upon heating, forinstance in long-term operation at high electrical powers, can expandwithout hindrance in the axial direction. Otherwise, undesirable strainsin the material of the discharge vessel could arise and/or the electrodecould sag. The aforementioned advantages of these variants are moreovernot limited in their advantageous effects to the mode of operation ofthe invention, but instead are fundamentally suitable for all lamps of asimilar type.

FIG. 2a shows a cross section through the discharge lamp of FIG. 1 takenalong lines A--A. The internal electrode 3 is located centrally, and twoelectrodes 4a, b are distributed symmetrically on the circumference ofthe outer wall of the discharge vessel 2.

The basic layout of the requisite voltage supply for the operationaccording to the invention of the discharge lamp 1 likewiseschematically shown in FIG. 1. The pulse train, that is, the shape andduration of the voltage pulses and the duration of the idle times, aregenerated in a suitably operated, or controlled pulse generator 10 andamplified by a following power amplifier 11. The pulse train is shownschematically as it appears at the internal electrode 3. A high-voltagetransformer 12 transforms the signal of the power amplifier 11 to therequisite high voltage. The lamp is operated with pulsed direct voltage.This involves negative square pulses as shown in FIGS. 3a. They have thefollowing parameters: pulse time Tp=2 μs, idle time T₀ =25 μs, voltageamplitude Up during Tp: -3 kV, and voltage amplitude U₀ during T₀ :0 V.

The inner wall of the discharge vessel is also coated with a layer 6 ofluminous substance. The UV radiation preferably emitted by the dischargein this exemplary embodiment is thus converted to the visible range ofthe optical spectrum, so that the lamp is suitable particularly forlighting purposes. This involves a three-band luminous substance havingthe following components: the blue component is BaMgAl₁₀ O₁₇ :Eu²⁺, thegreen component is Y₂ SiO₅ :Tb, and the red component is Y₂ O₃ :Eu³⁺.Thus a light yield of 37 lm/W is attained. At a color temperature of4000 K, an Ra>80 was attainable as the color repetition index. The VUVvacuum ultraviolet yield ascertained with the aid of the luminoussubstance is approximately 65%. Some other examples of fillings andoperating data of this lamp can be found from the following table. Init, p stands for the gas pressure, Up for the maximum value of thevoltage pulse, up for the maximum value of the voltage pulse referred tothe sparking distance (1.2 cm) and the pressure, and eta_(VUV) standsfor the VUV yield attained. The electrical power introduced was 18 W ineach case and the pulse duration T_(p) (length of time between rise andfall to approximately 10% of the maximum value in each case) wasapproximately 1.5 μs (at a half-value width of 1 μs), and the idle timeT₀ was approximately 27 μs.

                  TABLE                                                           ______________________________________                                                                     up in V/cm                                       p(Xe) in hPa                                                                           p(Ne) in hPa                                                                             Up in kV Pa      η.sub.VUV in %                       ______________________________________                                        100      --        2.41      0.200   55                                       133      --        2.39      0.150   60                                       200      --        2.95      0.123   65                                       200      733       3.50      0.031   60                                       ______________________________________                                    

FIG. 2b shows another exemplary embodiment. The internal electrode 3' islocated eccentrically in the vicinity of the inner wall and parallel tothe longitudinal axis of the cylindrical discharge vessel 2; theexternal electrode 4' is fixed diametrically opposite it on the outerwall. This arrangement is especially advantageous with cylindricaldischarge vessels of small cross section, because on the one hand thedischarge extends diametrically within the discharge vessel, and on theother the outer wall is covered with only a strip of conductive silveras an external electrode; that is, the radiating area is not furtherreduced by a second external electrode as in FIG. 2a.

In another exemplary embodiment in FIG. 2c, the internal electrode 3 islocated centrally inside the discharge vessel 2, as in FIG. 2a. Fourexternal electrodes 4'a, 4'b, 4'd, 4'e are mounted symmetrically,distributed over the circumference of the outer wall of the dischargevessel 2, so that this configuration is especially suitable fordischarge vessels of large cross section and hence with a large sheatharea. As a result, the discharge burns not only in a first plane as inFIG. 2a or 2b, but also in a further, second plane, and as a result thevolume of the discharge vessel 2 is utilized still better for radiationgeneration than is the case in the exemplary embodiments of Figs. 2a and2b.

In another embodiment, the inner wall of the rod lamp of FIG. 1 has notthe luminous substance coating 6 but instead a coating that reflects UVor VUV radiation--for instance, a coating of MgF₂, Al₂ O₃ or CaF₂ ; onlya narrow strip of the inner wall, preferably parallel to the lamp axis,is uncoated. The external electrodes are located such that the UV or VUVradiation can be emitted, unimpaired, through these strips. Thisembodiment is especially suitable for efficient VUV radiation ofelongated objects, for instance for purposes of illumination inlithography. In a preferred variant of this embodiment, the internalelectrode is replaced by a second external electrode. As a result, theUV or VUV radiation can be reflected unimpaired at the coating and beemitted to the outside through the striplike transparent region.

In FIG. 3a, a pulse shape of the voltage between the internal electrode(cathode) and external electrode (anode) that is preferred according tothe invention for the discharge that is dielectrically impaired on oneend is shown schematically. The voltage shape can deviate from that ofthe exemplary embodiment of FIG. 3a, as long as the voltage pulses atthe internal electrode begin with the negative sign and are separated byidle times.

FIG. 3b schematically shows a pulse shape whose polarity changes frompulse to pulse. It is suitable only for the discharge dielectricallyimpaired on both ends; the first pulse can begin with an arbitrarypolarity.

FIG. 4a shows the plan view and FIG. 4b the cross section throughanother embodiment of a discharge lamp dielectrically impaired on oneend, which can be operated by the novel method. This is an arearadiator, which has an upper radiating surface 7a and a lower radiatingsurface 7b parallel to it, to which surfaces the internal electrodes 3and external electrodes 4 are oriented at right angles and arranged inalternation such that a number of parallel discharge chambers 8 arecreated. Adjacent external and internal electrodes are each separated bya dielectric layer and a gas-filled discharge chamber 8, while adjacentinternal electrodes are separated only by a dielectric layer. The methodof operation according to the invention makes it possible toelectrically supply a plurality of parallel-connected discharge chambers8 with only a single voltage supply 13. The inner wall of the dischargevessel is coated with a luminous substance layer 6. The area radiator isequally attainable by putting together discharge chambers dielectricallyimpaired on both ends.

FIG. 5a shows the side view and FIG. 5b the cross section of a furtherembodiment of a discharge lamp. It is similar in its external form toconventional lamps with an Edison base 9 and can be operated by thenovel method. Inside the discharge vessel 2, an elongated internalelectrode 3 is centrally located, its cross section being shaped like asymmetrical cross, or plus sign. On the outer wall of the dischargevessel 2, four external electrodes 4'a, 4'b, 4'd, 4'e are mounted suchthat they face the four long sides of the internal electrode 3, and thedischarge structures thus burn substantially in two planes that are atright angles to one another and intersect at the longitudinal axis ofthe lamp.

In a further preferred variant of the above embodiment, the internalelectrode comprises a rod of special steel of circular cross section,with a diameter of 2 mm. It is located centrally axially inside acircular-cylindrical discharge vessel of 0.7-mm-thick glass. Thedischarge vessel has a diameter of approximately 50 mm, and on the endremote from the base it has a pump tip in which the end remote from thebase of the internal electrode is guided. The interior of the dischargevessel is filled with xenon at a pressure of 173 hPa. The externalelectrodes are formed by 12 strips, 1 mm wide and 8 cm long, ofconductive silver that are distributed axially parallel and uniformly onthe outer wall of the discharge vessel. The external electrodes areelectrically conductively joined to one another in the region of thebase by means of an annular strip of conductive silver attached to theouter wall. The inner wall of the discharge vessel is coated with alayer 6 of luminous substance. This is a three-band luminous substancehaving the blue component BaMgAl₁₀ O₁₇ :Eu²⁺, the green component LaPO₄:(Tb³⁺, Ce³⁺) and the red component (Gd, Y)BO₃ :Eu³⁺. A light yield of40 lm/W is thus attained. The color temperature is 4000 K, and the colorsite under the color standard table of CIE has the coordinates x=0.38and y=0.377. The courses over time of the voltage U(t), current I(t) andpower P(t) =U(t)·I(t) can be seen from FIG. 7 and--on a different timescale--FIG. 8. The maximum value of the voltage of the internalelectrode with respect to the external electrodes is approximately -4kV. The pulse duration (length of time at half the maximum value) andthe idle time are approximately 1.2 μs and 37.5 μs, respectively. InFIG. 8, four preliminary pulses of lesser amplitude are also clearlyvisible before the second primary pulse of the voltage source U(t). Ascan be learned from the corresponding courses of the current I(t) andpower P(t), no current flows during these preliminary pulses, andconsequently no electrical power is coupled into the gas. Suchpreliminary pulses are therefore harmless for the mode of operationaccording to the invention. At a pulse frequency of 25 kHz, a VUV yieldof approximately 65% is attained.

In another variant of the above embodiment, the discharge vesselcomprises material transparent to UV or VUV radiation, such asSUPRASIL^(R) --quartz glass (made by Heraeus Quarzschmelze GmbH). It issuitable as a VUV radiator, for instance in photochemistry. In a furthervariant, the internal electrode is coated with glass. This isadvantageous particularly when aggressive media, such as noble gashalides, are used, because in this way corrosion of the internalelectrode is averted.

FIGS. 9a, b show photographs of discharge patterns according to theinvention, generated with unipolar voltage pulses. FIG. 9a relates to adischarge dielectrically impaired on two ends. A circular-cylindricaltubular glass discharge vessel is provided on its outer wall with twodiametrically opposed, axially located striplike external electrodes.Inside the discharge vessel and in the connecting plane of the twoexternal electrodes, the greenish triangle-like discharges are arrangedin a row. The narrow apexes of the triangle-like discharge patterns eachbegin at the inner wall toward the cathode and widen until they meet theanode-side inner wall of the discharge vessel. FIG. 9b shows a dischargedielectrically impaired on one end. The discharge arrangement differsfrom that of FIG. 9a only by an additional metal rodlike internalelectrode. It acts as a cathode and is located centrally axially insidethe discharge vessel. From the surface of the internal electrode, thevarious triangle-like discharge patterns each widen toward one of thetwo external electrodes. Particular in FIG. 9b it can clearly be seenthat the patterns illuminate essentially uniformly diffusely. Only attheir narrow cathode-side end points do they each have a somewhatbrighter-illuminating region, which percentage wise is quite small.Moreover, the high uniformity is notable, both with respect to thedistance between the various patterns and with respect to the shape andlight density distribution of the various patterns in comparison withone another.

The great number of identical discharge patterns is in striking contrastto the photographic views of FIGS. 10a-d. These photographs, in thisorder, show the gradual transition to undesired discharge patterns. Thedischarge arrangement is equivalent to that of FIG. 9b. In FIG. 10a, afew triangle-like discharge patterns according to the invention canstill be seen. In the lower left region of this view of the dischargearrangement, a discharge has already developed that is similar in shapeto a Y. In the upper region of this view--somewhat to the left of thecenter of the image--a filamentlike, brightly illuminating discharge hasalready developed, at the expense of a few triangle-like discharges thatinitially are adjacent to them at the right. The increased light densityat the inner wall of the discharge vessel is an indication of a slidingdischarge in this region. The discharge region shown in FIG. 10b has astill-further reduced UV efficiency compared with FIG. 10a. The numberof discharge patterns originally present in this region has decreasedstill further. FIGS. 10c and 10d relate to a discharge dielectricallyimpaired on both ends (the discharge arrangement corresponds to that ofFIG. 9a) and on one end, respectively. In both cases, only afilamentlike discharge can now be seen. In the region of the anode, twostriplike sliding discharges can now be seen on the inner wall of thedischarge vessel. They open out like a Y to form a brightly illuminatingarclike structure. On the opposite cathode-side inner wall, thisstructure divides again into two similar striplike sliding discharges(FIG. 10c) or--in the case of discharge dielectrically impaired on oneend--ends at the cathode.

The invention is not limited to the exemplary embodiments indicated. Inparticular, various characteristics of various exemplary embodiments canbe suitably combined with one another.

We claim:
 1. A method to operate an incoherently emitting radiationsource, in particular a discharge lamp (1), from an electrical energysupply (10, 11, 12), by means of an electrically impededdischarge,wherein said radiation source comprises an at least partiallytransparent discharge vessel (2) of electrically nonconductive material;a gas filling (5) inside the vessel; at least two electrodes (3, 4)mounted in the vicinity of the gas filling (5); supply lines connectingsaid electrodes (3, 4) to the electrical energy supply (10-12); and adielectric layer disposed between at least one electrode (4) and the gasfilling (5), said method, in accordance with the invention, comprisingcausing the electrical energy supply (10-12) to furnish energy betweenthe electrodes (3, 4) in the form of a train of voltage pulses havingcharacteristic parameters, wherein any individual pulse (n) of the trainhas a voltage parameter U_(Pn) (t) and a pulse duration parameterT_(Pn), which pulse duration parameter is of the order of magnitude ofapproximately 1 ns to 5 μs; wherein each pulse (n) is separated from itssuccessor pulse (n+1) by an idle time parameter of duration T_(0n),which idle time is in the order of magnitude of approximately 500 ns to1 ms and with an idle time voltage parameter of U_(0n) (t); introducing,during the pulse duration T_(Pn), predominantly electrical effectivepower into the gas filling (5) by controlling the voltage U_(Pn) (t) ofsaid voltage pulse during said pulse duration; causing, during the idletime T_(0n) duration, the voltage U_(0n) (t) to be at a level to permitthe gas filling (5) to revert to a state which is similar to the stateof said gas filling prior to the particular preceding voltage pulseU_(Pn) (t); and controlling the parameters of the variablespulse voltagelevel U_(Pn) (t), pulse time duration T_(Pn), voltage during idle timeU_(0n) (t), idle time duration T_(0n), mutually relative to one anothersuch that, between the electrodes (3, 4), discharge structures ofcomparatively low current densities are created.
 2. The method of claim1, characterized in that the durations T_(0n) are selected such that themean value over time of the volume of an individual discharge structurebecomes maximal.
 3. The method of claim 1, characterized in that duringthe durations T_(Pn), when the voltage shapes U_(Pn) (t) occur betweenthe electrodes (3, 4), voltage values are selected the reignitionvoltage for the for supplying discharge.
 4. The method of claim 3,characterized in that the parameters of voltage U_(Pn) (t) and U_(0n)(t) and the durations T_(Pn) and T_(0n) are selected with respect to thefill pressure, the type of filling, the sparking distance between theelectrodes, the dielectrics, and the electrode configuration.
 5. Themethod of claim 4, characterized in that the voltage U_(Pn) (t) includesone or more of the following basic shapes, directly or approximately:triangular, square, trapezoidal, stairstep, arclike, parabolic,sinusoidal.
 6. The method of claim 5, characterized in that during thedurations T_(Pn), the maximum values for the voltage pulses Un_(Pn) (t)between the electrodes (3, 4) are of a level at least equivalent to thereignition voltage plus the voltage drop caused by the dielectric. 7.The method of claim 6, characterized in that the maximum values of thevoltage pulses are in the range between 0.01 and 2 V per centimeter ofsparking distance and per pascal of fill pressure.
 8. The method ofclaim 1, characterized by the step of reinforcing the development ofdischarge structures of comparatively low current densities bycontrolling the thicknesses of the dielectric layers and low relativedielectric constants.
 9. The method of claim 1, characterized in thatthe voltage of the energy source (10-12) is periodical.
 10. The methodof claim 1, characterized in that at least for one electrode, thedielectric layer is formed by a wall of the discharge vessel (2). 11.The method of claim 1, characterized in that the ratio of the area ofthe electrode surface in contact with the dielectric to thecircumference of this electrode surface is low.
 12. The method of claim1, wherein in case of the discharge dielectrically impeded at oneelectrode, the shape of the voltage pulse U_(Pn) (t) of thedielectrically unimpeded electrode or electrodes (3), measured withrespect to the dielectrically impeded electrode or electrodes (4) duringthe introduction of power, begins essentially with negative values,neglecting possible positive voltage peaks that are insignificant interms of the effective power introduction.
 13. The method of claim 1,characterized in that in the case of the discharge dielectricallyimpeded at one electrode, the shape of the voltage pulse U_(Pn) (t) ofthe dielectrically unimpeded electrode or electrodes (3), measured withrespect to the dielectrically impeded electrode or electrodes (4) duringthe introduction of power are exclusively negative, neglecting possiblepositive voltage peaks that are insignificant in terms of the effectivepower introduction.
 14. The method of claim 1, wherein a plurality ofdielectrically impeded electrodes are provided, said step of causingenergy to be supplied comprisesapplying unipolar or bipolar voltagepulses, or voltage pulses with alternating polarity between electrodesdielectrically impeded on two ends or terminals thereof.
 15. The methodof claim 1, characterized in that in the case where a plurality ofdielectrically impeded electrodes are used, bipolar voltage pulses areapplied between dielectrically impeded electrodes.
 16. The method ofclaim 1, characterized in that one or more electrodes, optionally of rodor strip shape, are disposed in the discharge vessel (2);and in that theelectrodes are located centrally or eccentrically, and optionally atleast one of the electrodes is dielectrically sheathed.
 17. The methodof claim 1, characterized in that one or more electrodes are locatedoutside the discharge vessel, and shaped in striplike form.
 18. Themethod of claim 1, characterized in that the discharge vessel (2)comprises a tube; one (3) of the electrodes is located in thelongitudinal axis and at least another electrode (4) is located on theouter wall of the vessel (2).
 19. The method of claim 1, wherein thedischarge vessel (2) is of a generally flat block-shaped structure,which is defined by side faces and two cover faces (7a, 7b), throughwhich the radiation essentially takes place, wherein said electrodesform internal and external electrodes (3) and (4), mounted at rightangles to the cover faces and, respectively, located to form a number ofparallel discharge chambers (8), which are located in a plane that isparallel to the cover faces (7a, 7b) of the generally flat block-shapedstructure, whereby said cover faces (7a, 7b) will form a radiatingplane; andwherein at least one gas-filled discharge chamber (8) and adielectric layer separates the electrodes (3, 4) each adjacent to oneanother and of different electrical potential.
 20. The method of claim19, characterized by dielectric layers separating the electrodes fromthe gas-filled discharge chamber.
 21. The method of claim 1,characterized in that the discharge vessel (2) is essentiallycylindrical and is provided on one end with a base (9),wherein one (3)of the electrodes is centrally located inside the discharge vessel, andis rod-shaped optionally fixed on one end; and wherein the otherelectrode (4) comprises at least one striplike electrode (4'a, 4'b, 4'd,4'e) located on the outer wall of the discharge vessel (2).
 22. Themethod of claim 21, characterized in that the central electrode (3) hasa circular cross section.
 23. The method of claim 1, characterized by aluminous substance (6) coating the walls that define the dischargechamber, at least partially.
 24. The method of claim 1, characterized inthat the operating pressure of the gas filling (5) is in the rangebetween 100 Pa and 3 MPa, and optionally is more than approximately 1kPa.
 25. The method of claim 1, wherein the electrical energy supply(10, 12) furnishes voltage pulses which, selectively, are unipolar, andwherein the discharge develops individual triangle-like dischargestructures;or, selectively, wherein the voltage supply furnishes voltagepulses of alternating polarity, and the discharge is dielectricallyimpaired on both electrodes, said discharge producing amirror-image-like superposition of two triangle-like dischargestructures that are essentially of hourglass shape; and including thestep of creating, by selectively varying said parameters, distancesbetween said individual discharges such that, in a limiting case, theentire discharge plane is radiating similar to a curtain-like structure.